1. Field of the Invention
The present invention relates to a microphone, and more particularly, to a piezoelectric microphone.
2. Description of the Related Art
Microphones are used to convert sound waves or ultrasonic waves into electric signals.
Examples of the microphones include a carbonic microphone, a piezoelectric microphone, a movable-coil microphone, a vibration-diaphragm microphone, a condenser microphone, and a semiconductor microphone. The carbonic microphone operates using the pressure-dependent electric resistance of carbon particles. The movable-coil microphone operates using a current induced by a vibration of a coil attached to a vibration coil. The vibration-diaphragm microphone operates using an induced current generating when a sound wave vibrates a ribbon-shaped diaphragm disposed in a magnetic field. The condenser microphone operates using the concept of a condenser in which thin-vibration plates (fixed electrodes) face each other at a close distance. The semiconductor microphone operates using a stress semiconductor having an electric resistance varying according to a mechanical force applied to the stress semiconductor.
FIGS. 1 and 2 illustrate a conventional piezoelectric microphone. In the conventional piezoelectric microphone, a lower electrode 20, a piezoelectric layer 30 formed of a piezoelectric material such as aluminum nitride AlN and piezoelectric ceramic, and an upper electrode 40 are sequentially formed on a protection layer 10 deposited on a silicon substrate (S).
The conventional piezoelectric microphone can be fabricated through a process illustrated in FIG. 3.
Referring to FIG. 3, in operation S10, the protection layer 10 is deposited on the silicon substrate (S). In operation S12, the lower electrode 20, the piezoelectric layer 30, and the upper electrode 40 are sequentially deposited on the protection layer 10. In operation S14, the backside of the silicon substrate (S) is etched by back-side etching to remove a center portion of the silicon substrate (S) until the bottom surface of the protection layer 10 is exposed.
The piezoelectric layer 30 generates a piezoelectric signal in proportion to an applied sound pressure, and the piezoelectric signal is transmitted to an external amplifier through the lower and upper electrodes 20 and 40.
Since the lower electrode 20, the piezoelectric layer 30, and the upper electrode 40 are formed into a simple parallel plate structure, the voltage level of the piezoelectric signal generated in response to the sound pressure is limited to below a predetermined value. As a result, it is difficult to increase the sensitivity of the piezoelectric microphone.